Flow Parallel Wire - Flow Sensor Insensitive to Fluid

Available online at www.sciencedirect.com

Procedia Engineering 25 (2011) 1189 – 1192

Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece

Flow Parallel Wire - Flow Sensor Insensitive to Fluid Properties C. Gerhardy*, W. K. Schomburg RWTH Aachen University, Konstruktion und Entwicklung von Mikrosystemen (KEmikro), Steinbachstraße 53 B, 52074 Aachen, Germany

Abstract

A flow parallel wire (FPW) with a center tap is used in a sensor for measuring the time of flow. A heater is placed upstream of the sensor. The upstream and downstream parts of the FPW are combined to a half bridge resulting in a large output peak when a heat pulse passes the FPW. The time between generating the heat pulse and recording the peak maximum is insensitive to the properties of the fluid. © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Keywords: Flow sensor, time of flow, flow parallel wire, insensitive to fluid properties

1. Introduction Nearly 30 years ago it was established that the flow velocity of liquids could be measured by detecting the time of flow (TOF) between generating a heat pulse and the arrival of this pulse at a sensor [1]. It was investigated whether flow measurements independent of the properties of the fluid are possible [2, 4]. But experiments showed that the measured time of flow was still a function of the fluid [3]. It was suggested that the different heat conductivity and viscosity of the fluids lead to a different transition time of the heat between heater and fluid and the fluid and the sensor.

* Christof Gerhardy Tel.: 0049-241-8028442; fax: 0049-241-8028442; E-mail address: [email protected].

1877-7058 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. doi:10.1016/j.proeng.2011.12.293

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C. Gerhardy and W.K. Schomburg / Procedia Engineering 25 (2011) 1189 – 1192

In a first attempt to eliminate the effect of heat transition, we measured the flow time ¨t of the temperature maximum between two sensor wires mounted downstream of a heater (cf. Fig. 1a, sensor A). This way, the heat transition from the heater to the fluid should have no influence on the measurement and heat transition from the fluid to the two wires should be similar. As an alternative solution, we arranged a flow parallel wire (FPW) in the center of a channel and combined the upstream and downstream parts of it to a half bridge as shown in Fig. 2. The goal was to create an output signal with a zero-crossing corresponding to the time when the center of the heat pulse arrives at the center tap of the wire. The fluids employed for testing were water, ethanol, and two different oils. The clearly different properties of the fluids are shown in Tab. 1. Table 1. Fluid properties Measured Fluids Properties

Water

Ethanol

Density ρ [kg/m³]

998

790

809

857

Viscosity Ș [mPa s]

1.0

1.2

3.25

49.3

0.598

0.165

0.15

0.15

Heat conductivity λ [W/(m K)]

Oil 1

Oil 2

2. Sensors and Measurements For both sensors the heater was formed by an 8 mm long copper coil with wire and coil diameters of 100 μm and 800 μm, respectively. The heater was driven by current pulses of 2 A with a duration of 100 ms. The flow was generated by a syringe pump. The sensor wires were made of 17.5 μm gold wire and the the flow channel was milled into polymethylmethacrylate with a cross-section of 1mm x 1mm. Δt

0,4

Sensor wires

ΔT

.

Flow

Wire 1

0,3

1

Time of flow Δ t [s]

Amplitude [V]

0,5

2

Heater

0,2

Water

3

2

Wire 2

Ethanol Oil 1

1

0,1

Theory

Oil 2

0

0 0

1

2 3 Time [s] (a)

4

5

0

10

20

30

Velocity v [mm/s] (b)

Fig. 1. (a) Sensor A with two sensor wires across the channel; (b) Measured curves of water, ethanol and two oils (sensor A)

The measured flow time between the two wires of sensor A remained a function of fluid properties (cf. Fig. 1b). Especially water leads to a much higher flow time as expected according to calculations. Sensor B was equipped with a sensor wire, 10 mm in length and arranged parallel to the flow in the center of a 55 mm long flow channel and the upstream and downstream parts of it were combined to a half

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bridge(Fig. 2 a and b). The output signal is shown in Fig. 2a. The time between the end of the heater pulse and maximum ¨tMax and zero crossing ¨t0 of the bridge output were recorded.

Sensor wires

ΔtMax Flow

Heater

Δt0

(a)

(b)

Fig. 2. (a) Sensor B employing two sensor wires parallel to the channel; (b) Photo of sensor B without a lid

Time of flow Δ t0 [s]

3,5

Time of flow Δ tMax [s]

The characteristic curves of sensor B are a much weaker function of fluid properties than the curves of sensor A. The flow time of the zero crossing ¨t0 and the maximum ¨tMax were plotted over the mean flow velocity (cf. Fig. 3 a and b, respectively). Every measurement was repeated four times and the mean value and standard deviation of the five values measured are shown in the figures.

Water Oil 1 Oil 2

2,5

Ethanol

Theory

1,5 0,5 0

10

20

Velocity v [mm/s] (a)

30

1,2

Water Oil 1 Oil 2

0,9

Ethanol Calculated

0,6 0,3 0

10

20

Velocity v [mm/s]

30

(b)

Fig. 3. (a) Time of flow ¨t0 for different fluids (sensor B); (b) Time of flow ¨tMax for different fluids (sensor B)

The time of zero crossing ¨t0 only differs for water by approximately 10 % from the other curves and the measuring of ¨tMax seems to be only a very weak function of fluid properties for low flow velocities. This is remarkable because the viscosities Ș of the fluids differ by a factor of up to 49 and their heat conductivities Ȝ by a factor of up to 4 (Table 1). Fig. 4a shows the times ¨tMax as a function of the inverse of the flow velocities of an arrangement of two FPWs 4.5 and 45 mm downstream of the heater. The combination of two FPWs allows measuring a larger flow range of approximately 0.01-0.5 m/s.

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To analyse the temperature cross sensitivity of sensor B, it was tested in a climatic chamber at three temperatures, using water. As seen in Fig. 4b, the surrounding temperature has only a very slight effect on the measured time of flow. 1,5

Water Δ tMax [s]

Time of flow Δ tMAX [s]

1

Ethanol

0,5

1

10°C

0,5

30°C 20°C

0

0 0

0,01

0,02 0,03 1/v [s/m m ] (a)

0,04

0,05

0

0,05

0,1

0,15

0,2

1/v [(s/mm)] (b)

Fig. 4. (a) Time of flow ¨tMax for water and ethanol (sensor B); (b) Temperature sensitivity of sensor B for water

3. Conclusions Measuring the time of flow of a heat pulse with a sensor wire parallel to the flow channel results in characteristic curves which are only a very weak function of fluid properties and temperature. Therefore, the flow parallel wire (FPW) allows flow measurements which are insensitive on fluid composition.

References [1] T.E. Miller, H. Small, “Thermal Pulse Time of Flight Liquid Flow Meter”, Anal. Chem. 54 (1982) 907 – 910. [2] T. Lammerink, et al., “Intelligent gas-mixture flow sensor”, Sensors and Actuators A 46 - 47 (1995) 380 - 384. [3] M. Ashauer, H. Glosch, F. Hedrich, N. Hey, H. Sandmaier, W. Lang, „Thermal flow sensor for liquids and gases“, Proc. MEMS (1998) 351 - 355, ISBN 0-7803-4412-X. [4] S. Cerimovic, et al., “Micromachined Flow Sensors Enabling Electrocalorimetric and TOF Transduction”, Eurosensors (2009) 132 - 135